This invention relates to a light wave distance measuring instrument for measuring a reflection turnaround time of a pulse of light to calculate a distance between a body of the instrument and an object, and more particularly to a light wave distance measuring instrument of the pulse type which is capable of removing an influence of any drift.
A conventional light wave distance measuring instrument of the pulse type measures a period of time required for a pulse of light to go from a body of the light wave distance measuring instrument and come back from a corner cube to measure a distance between the instrument body and the corner cube. However, since the velocity of light is very high, a high technique is required for direct measurement of such turnaround time. Thus, a light passage for reference is provided in the inside of the body of the light wave distance measuring instrument, and a period of time from a reference to reception of light based on an internal electric signal is measured for each of the internal reference light passage and an external light passage for distance measurement, and then a measured value obtained from the internal reference light passage is subtracted from a measured value obtained from the external distance measurement light passage to determine a reflection turnaround time to measure a distance between the instrument body and the corner cube. In other words, the conventional light wave distance measuring instrument of the pulse type is constructed such that instability factors occurring in the inside of the light wave distance measuring instrument may be removed by subtracting a measured value obtained from the internal reference light passage. Then, a light chopper which is a mechanical light passage changing over means is employed for such changing over between the light passages.
However, with such a conventional light wave distance measuring instrument of the pulse type as described above, measurement for the external distance measurement light passage and measurement for the internal reference light passage involve an interval of time from an electric reference signal to actual emission of a pulse of light from a light emitting diode. Such interval of time from a reference signal to actual emission of light is considered to involve such factors as
(A) a delay time and a drift from generation of a reference signal to a light emitting element driving circuit,
(B) a delay time and a drift of the light emitting element driving circuit, and
(C) a response time and a drift of the light emitting element.
Those factors can be reduced by subtracting a measured value obtained from the internal reference light passage from a measured value obtained from the external distance measurement light passage. However, since the conventional light wave distance measuring instrument employs a chopper which is a mechanical light passage changing over means, it has a problem that a time is required for such changing over operation and, if the frequency of changing over operations increased, then the measurement time increases accordingly. Further, the changing over frequency of the chopper must necessarily be set to several Hz, which results in production of a time difference in measurement between the internal reference light passage and the external distance measurement light passage. Consequently, there is a problem that an influence of such drifts as listed above cannot be eliminated.